1. Field of the Invention
The present invention relates to a protective relay system with a digital data processing unit for protecting power transmission lines, for example.
2. Description of the Prior Art
An example of a prior art static type mho relay as shown in FIG. 1 will first be described to explain the problems involved in the prior art. In the figure, CT designates a current transformer for transforming current I', flowing through the power transmission line TL, to be protected, into a current I. The current I is converted into a resultant vector IZ as the product of the current I and a replica impedance Z simulated with respect to the line impedance of the transmission line TL by a converting circuit IM. The resultant vector IZ is applied to the (+) input terminal of a first comparator 1. PT is a potential transformer for transforming the voltage V' on the BUS into a voltage V. The transformed voltage V is supplied to the (-) input terminal of the first comparator 1 and a series resonant circuit RC. The series resonant circuit RC is for storing the voltage V of two or three cycles before a fault occurs in the transmission line at the closest possible point to the fault. The output voltage V.sub.p of the resonant circuit RC will be referred to as a polarity voltage and is supplied to the (+) input terminal of a second comparator 2. The (-) input terminal of the second comparator 2 is kept at a substantially zero potential. This zero potential may be a voltage minutely DC biased in order to secure a stable operation of the relay under a no input condition. The output signals S(IZ-V) and S(V.sub.p) of the first and second comparators 1 and 2 represent the phases of the signals (IZ-V) and V.sub.p, respectively. The signals S(IZ-V) and S(V.sub.p) are compared in phase with each other by an AND circuit 3. The output signal from the AND circuit 3 is applied to a time duration measuring circuit 4 for measuring the time duration of the output signal from the AND circuit 3 as to whether it exceeds a given value, for example, that corresponding to an electrical angle of 90.degree..
When the mho relay shown in FIG. 1 is used for a short circuit distance relay installed between the R and S phases of three phase transmission lines, an electrical quantity I is replaced by (I.sub.R -I.sub.S) as a vector difference between the R- and S-phase currents I.sub.R and I.sub.S and another electrical quantity V is replaced by the R-S line voltage V.sub.RS.
Assume now that an R-S short fault takes place in the operating range of the mho relay on the power transmission line TL. In this case, the time duration of a logical "1" state produced from the AND circuit 3 exceeds a setting time duration of the time duration measuring circuit 4. As a result, the circuit 4 produces a protection output signal "1" allowing the circuit breaker to trip. On the other hand, when such a fault takes place outside the operating range of the mho relay, the circuit 4 produces no protection output signal "1".
FIG. 2 shows a vector diagram illustrating a characteristic of the above-mentioned mho relay. As seen, when an angle .theta. defined by vectors (IZ-V) and V.sub.p is within 90.degree., the relay operates or produces the protection output signal "1". For protecting three phase transmission lines by the above-mentioned protective relays, three relays with the same structure and characteristic must be used.
For expanding the operating range of the mho relay (or increasing the replica impedance) when the mho relay is used to protect power transmission lines, the load impedance Z may be contained in the operating range of the relay. This is undesirable. This undesirable phenomenon can be avoided by using a combination of the mho relay and a blinder relay with an ohm characteristic. The characteristic of the blinder relay is illustrated in FIG. 3. The blinder relay operates when an angle .theta. defined by the vectors (IZ.sub.r -V) and IZ.sub.R is within 90.degree. to produce the protection output signal. The basic construction of the blinder relay is the same as that of the mho relay when vectors IZ and V.sub.p are replaced by vectors IZ.sub.r and IZ.sub.R, respectively.
As described above, in the prior art protective relay system, there are drawbacks that the protective relays constituting individual circuits with the same or similar arrangements must be provided for the respective phases in the power system to be protected in a one-to-one correspondence manner. The result is that the protective relay system is bulky.